Manufacturing a semiconductor wafer according to the process time by process tool

ABSTRACT

A semiconductor process for manufacturing a wafer. First, a previously predicted process rate and a previously measured process rate are provided by a process tool. Next, a presently predicted process rate is obtained by a first linear equation having a first variable weighting factor using the previously predicted process rate and the previously measured process rate as variables. Next, a process time is obtained according to the presently predicted process rate and a predetermined process target to input to the process tool. Finally, the wafer is manufactured according to the process time by the process tool.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor technology, and moreparticularly to a method to predict the removal rate of chemicalmechanical polishing to precisely control the polishing time for wafers.

2. Description of the Related Art

As semiconductor devices are scaled down, the importance of chemicalmechanical polishing (CMP) to the fabrication process increases.Generally, a chemical mechanical polishing tool includes a polishingdevice, a cleaning device, and a measurement device. The polishingdevice is positioned above a rotatable circular platen or table on whicha polishing pad is mounted. However, as the removal rate of CMPdecreases with consumption of the polishing pad, it is difficult tocontrol polishing time for polishing wafers. In order to control thepolishing time for wafers, it is necessary to estimate the removal rateof the wafer. That is, the precise polishing time can be input to theCMP tool by properly predicting the removal rate of the wafers. As aresult, the desired polishing thickness of layers to be polished on thewafers can be obtained.

FIG. 1 is a flow chart for controlling chemical mechanical polishingaccording to the prior art. First, in step S1, a measurement devicemeasures the pre-polishing thickness of the control wafer. Next, in stepS2, the control wafer is placed in the CMP tool to polish with polishingtime input. Next, in step S3, the measurement device then measures thepost-polishing thickness of the control wafer. Next, in step S4, aremoval rate is obtained by the predetermined polishing time and thecontrol wafer's thickness difference between pre-polishing andpost-polishing. Next, in step S5, the measurment device measures thethickness of a layer to be polished on the product wafer. Next, in stepS6, a new polishing time is determined by the removal rate of thecontrol wafer and the desired polishing thickness of the layer to bepolished on the product wafer. Next, in step S7, the product wafer ispolished according to the polishing time. Next, in step S8, thethickness of the product wafer is measured. Next, step S9 determineswhether the next product wafer is to be polished. If not, the CMP isfinished, as indicated at step S10. If the next product wafer is to bepolished, steps S5 to S8 are repeated. The determining of the polishingtime, as indicated in step S6, accords to the removal rate of theprevious wafer or lot thereof and the desired polishing thickness of thepresent wafer or lot of the product wafers. In general, a simple linearcombination between the previously measured removal rate of the controlwafer and the previously predicted removal rate is used to predict apresent removal rate. Thereafter, a polishing time for the product wafercan be obtained according to the present removal rate.

However, since the variation resulting from the process incoming noiseand the removal rate decay due to the consumption of the polishing pad,the polishing time obtained by the mentioned method is not precise. As aresult, underpolishing occurs in the product wafers. Accordingly, somemethods, such as exponentially weighted moving average (EWMA) andpredictor-corrector controller (PCC), have been suggested for preciselypredicting the removal rate to obtain the polishing time. Since thesemethods use a constant weighting factor, CMP consumable problems cannotbe overcome. That is, the polishing time cannot be precisely controlled.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide asemiconductor process for manufacturing at least one wafer toeffectively control the process time by variable weighting factors.

According to an aspect of the invention, there is provided asemiconductor process for manufacturing at least one wafer. First, apreviously predicted process rate and a previously measured process rateare provided by a process tool. Next, a presently predicted process rateis obtained by a first linear equation having a first variable weightingfactor using the previously predicted process rate and the previouslymeasured process rate as variables. Next, a process time is obtainedaccording to the presently predicted process rate and a predeterminedprocess target to input to the process tool. Finally, the wafer ismanufactured according to the process time by the process tool. Thefirst linear equation and the first variable weighting factor are:RR*(t)=W1(t)×RR(t−1)+(1−W1(t))×RR*(t−1), and W1(t)=W1+γ1 ^(t), 0<γ1<1.Where W1 and γ1 are experienced constants.

Moreover, in the semiconductor process, a previous adjustment valueD(t−1) is further provided. Thereafter, a present adjustment value D(t)is obtained by a second linear equation using the previous adjustmentvalue D(t−1) and the difference of the presently predicted removal rateand the previously predicted removal rate as variables. The presentlypredicted removal rate is modified by the present adjustment value D(t).The second linear equation has a second variable weighting factor, inwhich the second linear equation and the second variable weightingfactor are: D(t)=W2 (t)×(RR*(t−1)−RR*(t))+(1−W2(t))×D(t−1), andW2(t)=W2+γ2 ^(t), 0<γ2<1. Where W2 and γ2 are experienced constants.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be more fully understood by reading thesubsequent detailed description in conjunction with the examples andreferences made to the accompanying drawings, wherein:

FIG. 1 is a flow chart for controlling chemical mechanical polishingaccording to the prior art.

FIG. 2 is a sectional diagram showing an exemplary chemical mechanicalpolishing tool.

FIG. 3 is a flow chart for controlling chemical mechanical polishingaccording to the present invention.

FIG. 4 is a graph showing the relation between the removal rate and thepad lifetime according to the prior art and the invention, respectively.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 is an exemplary chemical mechanical polishing tool. The chemicalmechanical polishing tool 5 includes a wafer carrier 10 and a polishingpad 30. The carrier 10 is positioned above the polishing pad 30 to fixthe wafer 20 and to transport it onto the polishing pad 30. Thepolishing thickness d of the wafer 20 is controlled by a controller (notshown) in the CMP tool 5 and determined by a polishing time obtainedfrom predicting a removal rate.

FIG. 3 is a flow chart for controlling chemical mechanical polishingaccording to the present invention. First, a presently measured processrate such as a removal rate is necessary to provide by a process toolsuch as a CMP tool. For example, in step S21, wafers such as controlwafers or product wafers are polished by CMP tool. Thereafter, in stepS22, an initial measured removal rate RR(0) is obtained by thepredetermined process time such as polishing time and thicknessdifference of the wafer between pre-polishing and post-polishing. Thatis, RR(0)=(Pre(0)−Post(0))/PT(0), where the Pre(0) represents thepre-polishing thickness, Post(0) represents the post-polishingthickness, and the PT(0) represents the predetermined polishing time.

Hence, two equations (RR(t) and RR*(t)) are defined, where t representslot number of wafers. RR(t) represents a presently measured removal rate(RR(t)=(Pre(t)−Post(t))/PT(t)). It must be obtained by measuring thethickness of the wafers. RR*(t) represents a presently predicted removalrate. RR*(t) is obtained by previously measured removal rate RR(t−1) andpreviously predicted removal rate RR*(t−1), and used for obtaining thepolishing time for the next lot of wafers. Hence, and RR*(0) is equal toRR(0).

Next, in step S23, the presently predicted removal rate RR*(t) iscalculated by linear equation (1) using RR(t−1) and RR*(t−1) asvariables, as described below. In addition, W1(t) is a variableweighting factor, which varies with the lot number (t), and representsequation (2) as described below.

RR*(t)=W 1(t)×RR(t−1)+(1−W 1(t))×RR*(t−1)  (1)

W 1(t)=W 1+γ1 ^(t), 0<γ1<1.  (2)

W1 and γ1 are experienced constants. For example, W1 is equal to 0.3,and the γ1 is equal to 0.2.

As a result, the process variation due to the process incoming noisewith increasing lot number (t) (increasing the consumption of thepolishing pad) can be effectively eliminated.

Next, in the optional step S24, a present adjustment value D(t) isobtained by linear equation (3) using D(t−1) and RR*(t−1)−RR*(t) asvariables to modify the presently predicted removal rate RR*(t), asdescribed below. D(t−1) represents a adjustment value of the precedinglot of wafers. In addition, W2(t) is a variable weighting factor, whichvaries with the lot number (t), and represents equation (4) as describedbelow.

D(t)=W 2(t)×(RR*(t−1)−RR*(t))+(1−W 2(t))×D(t−1)  (3)

W 2(t)=W 2+γ2 _(t), 0<γ2<1.  (4)

W2, and γ2 are experienced constants. For example, W2 is equal to 0.3,and the γ2 is equal to 0.2. In addition, the initial adjustment valueD(0) is equal to 0 or other experienced constant.

The adjustment value D(t) is used to predict the drift value of theremoval rate, and eliminating the drift value from predicted removalrate RR*(t). The predicted removal rate after modifying RR**(t) is thedifference of the predicted removal rate RR*(t) and the adjustment valueD(t)(RR**(t)=RR*(t)−D(t)).

Next, in step S25, a process target, such as polishing thickness of alayer to be polished on the wafer, is determined. Thereafter, polishingtime PT(t) can be calculated according to the predetermined polishingthickness and the predicted removal rate RR*(t) or the predicted removalrate after modifying RR**(t)(PT(t)=polishing thickness/RR*(t) orPT(t)=polishing thickness/RR**(t)), and then input it to the CMP tool.

Next, in step S26, the next lot of wafers is polished according to thepolishing time by the CMP tool. In addition, it can obtain pre-polishingthickness Pre(t) of a layer to be polished on the product wafer by ameasurement device.

Next, in step S27, whether the next lot of product wafers is polished isdetermined. For example, estimating whether the polishing pad hasprocessed excess wafers. If so, step S28 commences, and the CMP breaksoff for preventive maintenance (PM). If not, proceeding to step S29,post-polishing thickness Post(t) is measured by the measurement device,and the measured removal rate RR(t) of the present lot of wafers can beobtained by the relation: RR(t)=(Pre(t)−Post(t))/PT(t). Thereafter,steps S23 to step S26 are repeated.

FIG. 4 is a graph showing the relation between removal rate andpolishing pad lifetime according to the prior art and the invention,respectively. “A” represents a relational curve between removal rate andpad lifetime according to the present invention. “B” represents arelational curve between removal rate and pad lifetime according to theprior art (EWMA). “C” represents a relational curve between removal rateand pad lifetime according to more prior art (PCC). According to thepresent invention, since two variable weighting factors (W1(t) andW2(t)) are used for effectively eliminating the variation due to theprocess incoming noise and the drift value of the removal rate, a smoothcurve A can be obtained. On the contrary, in the prior art (EWMA andPCC), since constant weighting factors are used, vibration and unstablecurves (B and C) are obtained. That is, according to the presentinvention, an ideal curve to predict removal rate can be obtained toprecisely control the polishing time.

Finally, while the invention has been described by way of example and interms of the preferred embodiment, it is to be understood that theinvention is not limited to the disclosed embodiment. On the contrary,it is intended to cover various modifications and similar arrangementsas would be apparent to those skilled in the art. Therefore, the scopeof the appended claims should be accorded the broadest interpretation soas to encompass all such modifications and similar arrangements.

What is claimed is:
 1. A semiconductor process for manufacturing atleast one wafer, comprising steps of: providing a previously predictedprocess rate and a previously measured process rate by a process tool;obtaining a presently predicted process rate by a first linear equationhaving a first variable weighting factor using the previously predictedprocess rate and the previously measured process rate as variables;obtaining a process time according to the presently predicted processrate and a predetermined process target to input to the process tool;and manufacturing the wafer according to the process time by the processtool.
 2. The semiconductor process as claimed in claim 1, wherein theprocess tool has a polishing pad.
 3. The semiconductor process asclaimed in claim 2, wherein the predetermined process target is apolishing thickness of a layer to be polished on the wafer.
 4. Thesemiconductor process as claimed in claim 2, wherein the process time isthe polishing time of the wafer.
 5. The semiconductor process as claimedin claim 2, wherein the previously predicted process rate is apreviously predicted removal rate RR*(t−1), and the previously measuredprocess rate is a previously measured removal rate RR(t−1).
 6. Thesemiconductor process as claimed in claim 5, wherein the first linearequation and the first variable weighting factor are: RR*(t)=W1(t)×RR(t−1)+(1−W 1(t))×RR*(t−1); and W 1(t)=W 1+γ1 ^(t), 0<γ1<1. WhereW1 and γ1 are experienced constants.
 7. The semiconductor process asclaimed in claim 6, wherein the W1 is 0.3.
 8. The semiconductor processas claimed in claim 6, wherein the γ1 is 0.2.
 9. The semiconductorprocess as claimed in claim 5, further comprising steps of: providing aprevious adjustment value D(t−1); obtaining a present adjustment valueD(t) by a second linear equation using the previous adjustment valueD(t−1) and the difference of the presently predicted removal rate andthe previously predicted removal rate; and modifying the presentlypredicted removal rate by the present adjustment value D(t).
 10. Thesemiconductor process as claimed in claim 9, wherein the second linearequation has a second variable weighting factor.
 11. The semiconductorprocess as claimed in claim 10, wherein the second linear equation andthe second variable weighting factor are: D(t)=W2(t)×(RR*(t−1)−RR*(t))+(1−W 2(t))×D(t−1); and W 2(t)=W 2+γ2 ^(t),0<γ2<1. Where W2 and γ2 are experienced constants.
 12. The semiconductorprocess as claimed in claim 11, wherein the W2 is 0.3.
 13. Thesemiconductor process as claimed in claim 11, wherein the γ2 is 0.2. 14.A semiconductor process for polishing at least one wafer, comprisingsteps of: providing a previously predicted removal rate RR* (t−1) and apreviously measured removal rate RR(t−1) by a process tool having apolishing pad; obtaining a presently predicted removal rate RR*(t) by afirst linear equation having a first variable weighting factor using thepreviously predicted removal rate and the previously measured removalrate as variables; obtaining a polishing time according to the presentlypredicted removal rate and a predetermined polishing thickness to inputto the process tool; and polishing the wafer according to the polishingtime by the process tool.
 15. The semiconductor process as claimed inclaim 14, wherein the first linear equation and the first variableweighting factor are: RR*(t)=W 1(t)×RR(t−1)+(1−W 1(t))×RR*(t−1), and W1(t)=W 1+γ1 ^(t), 0<γ1<1. Where W1 and γ1 are experienced constants. 16.The semiconductor process as claimed in claim 15, wherein the W1 is 0.3.17. The semiconductor process as claimed in claim 15, wherein the γ1 is0.2.
 18. The semiconductor process as claimed in claim 14, furthercomprising steps of: providing a previous adjustment value D(t−1);obtaining a present adjustment value D(t) by a second linear equationusing the previous adjustment value D(t−1) and the difference of thepresently predicted removal rate and the previously predicted removalrate as variables; and modifying the presently predicted removal rate bythe present adjustment value D(t).
 19. The semiconductor process asclaimed in claim 18, wherein the second linear equation has a secondvariable weighting factor.
 20. The semiconductor process as claimed inclaim 19, wherein the second linear equation and the second variableweighting factor are:  D(t)=W 2(t)×(RR*(t−1)−RR*(t))+(1−W 2(t))×D(t−1),and W 2(t)=W 2+γ2 ^(t), 0<γ2<1. Where W2 and γ2 are experiencedconstants.